CROSBY : ORIGIN OF ESKERS. 
389 
extremely low gradient and scores and hundreds of miles from the 
nearest mountains and these never more than one third as lofty as 
the St. Elias range, also became stagnant and free from crevasses 
across a marginal zone of considerable breadth. This point of view 
seems to demand not only great thickness of the continental ice 
sheet in its prime or before it ceased to move, but also overriding 
alike of the immature and of the old and wasted mai-gin as a means 
of inducing and renewing motion in the latter, and this overriding 
must involve the transfer of englacial drift in great volumes to higher 
levels in the ice than many glacialists have heretofore been willing 
to accept. 
Glacial potholes, it may be noted further, are, in general, far more 
likely to be the products of subglacial streams than of moulms^ for 
they lack the elongation which the latter explanation requires ; and 
the subglacial stream may or may not have originated in a moulin. 
The assumption that it did is by no means necessary. That the 
glacial potholes were in general formed after the ice became abso¬ 
lutely and finally stagnant must be obvious to any one who has 
studied them in the field and noted the perfectly normal and ungla¬ 
ciated condition of their rims. 
To summarize, it appears probable that eskers were formed in con¬ 
nection with the sluggish or wholly stagnant marginal portion of 
the waning ice sheet, after the liberation by basal melting of all that 
part of the ground moraine, including drumlins, showing evidence of 
having been pressed down and compacted by the movement as well 
as by the dead weight of the ice, and after the upper part of the 
englacial drift had become superglacial through the superficial melt¬ 
ing or ablation of the ice. Such crevasses as may have survived the 
cessation of flow, or resulted from local subsidence due to basal 
melting, were probably closed by drift washed into them from 
above. The general absence of modified drift interstratified with 
the till is an indication that subglacial streams, or a concentrated 
flow of subglacial waters, did not exist before the ice became stag¬ 
nant, the only explanation that suggests itself being that while the 
ice was in motion it would cause the drift to rise and fill any channel 
opened above it, a principle which is not operative beneath alpine 
glaciers (and possibly not beneath the Malaspina glacier) because 
they have long since swept their channels free of drift. 
